INT-3538: Docs for Scatter-Gather

JIRA: https://jira.spring.io/browse/INT-3538

Phase #1: `Scatter-Gather`

S-G Doc Polishing

INT-3538 Add `requires-reply` to `scatter-gather`

Final S-G Doc Polish

spring-integration-core/src/test/java/org/springframework/integration/scattergather/config/ScatterGatherParserTests.java

@@ -64,6 +64,7 @@ public class ScatterGatherParserTests {
 
 		Object reaper = this.beanFactory.getBean("reaper");
 		assertSame(gatherer.getMessageStore(), TestUtils.getPropertyValue(reaper, "messageGroupStore"));
+		assertTrue(TestUtils.getPropertyValue(scatterGather, "requiresReply", Boolean.class));
 	}
 
 	@Test